Calender roll with &#34;bi-axially oriented&#34; polymer segments



Math 24, 1970 A. GREGERSEN ETAL 3, 0 CALENDER ROLL WITH "Bi-AXIALLY ORIENTED" POLYMER SEGMENTS Filed March 22, 1968 INVENTORS ALV GREGERSEN 8 KARL-OLOF SELLDEN LARSSON their ATTORNEYS United States Patent US. Cl. 29132 6 Claims ABSTRACT OF THE DISCLOSURE A calender roll comprising a central core and a roll filling composed of discs fitted on the core and compressed together to form an essentially solid body. The discs are made of a polymeric sheet material having a biaxially oriented molecular structure.

BACKGROUND OF THE INVENTION This invention relates to a so-called filled calender roll of a type comprising a core, which is usually made of steel, and a plurality of discs made from sheet material and assembled onto the core and compressed together in the axial direction into an essentially solid body.

Filled calender rolls of this type are used as sof rolls in calenders for calendaring of various sheet materials, e.g., paper, to high surface finish. Such a calender comprises several rolls, soft as well as hard, the latter usually of steel, arranged to form press nips constituted by one soft roll and one hard roll.

In order to obtain a calendering effect which will give a material being calendered a high surface finish, it is desirable to create a high nip pressure between a soft roll and a hard roll. However, the magnitude of high pressure in the nip is limited in previously known calenders for the following reasons.

A soft roll provides a low per unit area pressure (overall pressure) in the nip in proportion to the force per unit of dimension (linear pressure) between the soft roll and a hard roll, and therefore, if the use of a softer roll is desired and at the same time a given overall pressure in the nip is to be established, the linear pressure between the soft roll and the hard roll must be increased. The increased linear pressure and the lower resistance of the soft roll to deformation combined to produce greater deformation of the soft roll and a consequent generation of heat in the soft roll, caused by the deformation work of the material. Inasmuch as the deformation work and resulting generation of heat take place not only at the surface of the soft, filled roll but also to a degree in the interior portions and inasmuch as the heat emission from the filled roll essentially occurs only from its cylindrical surface, mainly through heat absorption and conduction away by the web being calendered, a substantial rise in temperature will occur in the filled roll just beneath its surface, particularly when the material in the discs used in the filled roll structure has a relatively low heat conductivity.

To obtain a high production rate, the calender rolls must be run at a high peripheral speed. However, a high 3,501,823 Patented Mar. 24, 1970 "ice roll speed will also contribute to a greater deformation work per unit of time and a greater generation of heat in the material of the soft roll.

Soft filled rolls previously proposed and used in the art are usually built up of discs made of thin layers of paper. The paper used for the discs is often treated to provide better resistance against the detrimental effects of high temperatures, such as by the admiture of asbestos in the composition or the use of suitable chemical substances. Experiments have been performed using paper layers having thin coatings of metal to improve heat con duction out of the roll, and the similar concept of using thin discs of metal alternating with the paper discs has also been tried.

In soft filled rolls constructed of paper discs pyrolysis of the paper material of the discs occurs at a temperature below 200 C. This pyrolysis of the paper material causes an increase in the hysteresis losses in the paper, through which the generation of heat causes an ever increasing temperature rise. Finally, the paper material starts carbonizing, and the roll is then rapidly destroyed.

The tendency for soft filled rolls constructed as known heretofore to be destroyed by the generation of heat in the filling material acts as a definite limit on both the maximum overall nip pressure and the maximum roll speed in a calender. Moreover, the rolls known heretofore suffer from the further disadvantage of having a surface which is damaged or destroyed mechanically rather easily, which makes it necessary to regrind them or to provide them with new filling material after only relatively short service time.

Experimental investigations have shown that in running a calender roll at peripheral speeds of between 200' and 700 m./min. and a linear pressure in the roll nips of between 50 and 250 kp./cm., the maximum temperature of a roll can be calculated fairly closely by the formula,

Max Surface R where:

T is the maximum temperature in the roll in C.;

Tsmface is the surface temperature of the roll in C.;

P is the linear pressure in the nip in N/m.;

C is the peripheral speed of the roll in m./sec.;

H is the deformation work hysteresis loss in the nip;

A is the heat conductivity of the filling material of the roll in W/m. C.

E is the modulus of elasticity in N/m. and

R is the radius of the roll in m.

From the above equation it will be observed that an increase in the linear pressure, an increase in the roll speed and an increase in the softness of the roll (i.e., a reduction of the modulus of elasticity of the roll), all of which are desirable for better calendering and increased production, will result in higher temperature build-up in the soft filled roll.

An increase in the heat conductivity of the filling material of the roll and an increase in the radius of the roll will lead to a lower maximum temperature in the roll. Attempts to increase the heat conductivity by providing paper discs with metal coatings have, however, so far not given satisfactory results; the metal coating is quickly destroyed because its modulus of elasticity differs substantially from that of the paper material. An increase of the radius of the roll for a given linear pressure results in a lower maximum pressure in the nip and, consequently, a reduced calendering effect.

It is also clear that a reduction in the temperature build-up in a soft filled roll can be achieved by reducing the hysteresis loss caused by the deformation of the roll in the nip.

SUMMARY OF THE INVENTION There is provided by the present invention a soft filled calender roll which can be used in a roll nip with high pressure and/or with a high peripheral speed without being destroyed by heat and, moreover, has a high resistance to mechanical destruction. The roll of the invention also has a low hysteresis loss and the physical and mechanical properties suitable for calendering purposes. The above-mentioned and other advantages of a filled calender roll, according to the invention, are obtained by constructing the roll with discs made from polymer sheets in which the molecules are biaxially oriented in the planes of the sheets.

More particularly, a calender roll, according to the invention, comprises a roll filling made of discs of a polymer sheet material, the sheet material being formed in such a way as to produce a structure in which the molecules are biaxially oriented. The disc material may be selected from various polymers including polyesters, polyolefins, polystyrenes and polyamides. The biaxial molecular structure of the sheet is obtained by stretching a sheet formed by conventional methods in at least two direc tions while it is at an elevated temperature. The manufacturing techniques for producing a sheet having a biaxially oriented molecular structure, which structure is also essentially crystalline, are well known and do not, per se, constitute a part of the invention. Therefore, detailed description of this aspect is not necessary.

A roll filling, according to the invention, constituted by discs of biaxially oriented polymer sheets will generally have a deformation work hysteresis loss which is less than 0.1, while the modulus of elasticity will be about the same as for a roll filling of conventional paper material. By virtue of the low hysteresis loss, the temperature rise in the roll, even at very high nip pressures and high peripheral speeds will not reach a point so high that the roll filling is damaged. The elastic properties of the roll filling are such that the roll surface is resistant to damage under even such severe conditions as running defective paper through the calender.

The use of a polymer sheet material having a biaxially oriented molecular structure as the material for discs in a filled roll structure offers the further advantage of not requiring the discs to be carefully oriented, which, as is well known, is necessary with paper discs because, due to the orientation of the fibers, they are anisotropic. Such orientation of conventional paper discs usually requires special equipment. The fact that a roll according to the invention can be run with high linear nip pressure also makes it unnecessary to use as many rolls in a calender machine as are required with conventional rolls to obtain a given calendering effect.

DESCRIPTION OF EXEMPLARY EMBODIMENT Q For a better understanding of the invention, reference may be made to the following description of an exemplary embodiment, taken in conjunction with the figure of the accompanying drawing, which is a pictorial view of a portion of a roll, according to the invention.

Referring to the drawing, the roll consists of a core 1, which is made of steel or some other appropriate metal,

.4 illustrated in the drawing in positions in which they are only partway installed on the core 1, such illustration being principally for the purpose of more clearly indicating the roll filling structure.

The plurality of discs 5 making up the roll filling 2 are assembed onto the core 1 and compressed in the axial direction, using a press device of a type known in the art to form an essentially solid body, retainer heads 4 are assembled on the core and the assembly secured in place by locking nuts 3. Only one such head and its retainer nut 3 are shown in the drawing, but it will be understood that an identical head and nut are installed at the opposite end of the core to retain the opposite end of the roll filling 2. After the roll filling discs 5 are assembled and secured in place, the roll filling structure 2 is turned and ground to the required diameter and surface finish.

According to the invention, as described in some detail above, each of the plurality of discs 5 is cut from a sheet of polymeric material which has been stretched in at least two directions and under elevated temperature conditions such as to produce a structure in which the molecules are biaxially oriented in the major planes of the sheets. Various polymers including polyesters, polyolefins, polystyrenes and polyamides are suited for the roll filling, particularly for their properties of relatively low deformation work hysteresis loss when of biaxial molecular structure and moduli of elasticity which are generally of the same order as those of roll filling of conventional paper material; thus, they are well suited for replacement use in existing calenders. They also have high strength and toughness. The discs 5 may be assembled onto the core 1 without paying any particular attention to their orientation, the result being a random orientation of the biaxial molecular structure.

Some examples of polymers suitable for the filled calender rolls according to the invention are given below. Sheets of each polymer are processed in a manner producing a biaxial molecular structure. Desirably, the stretching producing the biaxial structure is equal in both directions so that the properties are equal in both directions, a condition which may be described as balanced biaxial orientation.

EXAMPLE 1 Polyethylene terephthalate (Melinex S, from ICI) Density: 0.91 g./cm.

Modulus of elas.: 3.6 10 p.s.i.

Tensile strength: 25,000-30,000 p.s.i.

EXAMPLE 3 Polycapramide nylon (Caprolan, Allied Chemical Co.)

Density: 'l.0 g./cm.

Modulus of elas.: 1 1.1 X 10 Tensile strength: 1 7500ll,500 p.s.i.

1 Not biaxlally oriented as tested.

EXAMPLE 4 Polystyrene (Polyflex Clear, from Sidaplex Belgium, Monsanto) Density: 1.05 g./cm. Modulus of elas.: 4.5 X 10 Tensile strength: 10,500 p.s.i.

As discussed above, an important aspect of the invention is the property of the biaxially oriented polymeric filling materials of a low deformation work hysteresis loss, and as little data on this property is available in the literature, it is useful to discuss here in more detail vantages and results not obtained with conventional paper rolls. Reference may be made to Table II immediately below for comparative data which demonstrates the improved results provided by a polymer filled roll. Note particularly the low deformation Work hysteresis loss and what this term means in connection with calender rolls. 5 maximum roll temperature.

TABLE II.SUMMARY OF PHYSICAL DATA FOR VARIOUS FILLED CALENDER ROLLS Nip deforma- Running of the rolls in the exptl. calender Dynamic tion work Surface modulus of hysteresis Linear Peripheral hardness, elasticity loss H, pressure, roll velocity, Max. filled roll Filling material Shore D X10 kp./cn:i. percent kpJcm. m./min. Time, min. temp, C.

Paper, 80% cotton, wool 83 2. 60 40 150 500 20 108 Paper, 80% cotton, 20% wool 87. 3 3. 37 150 500 20 66 250 300 10 1 160 Paper, 60% cotton, 40% wool 83 N.A. N.A. 250 500 3 1 208 Paper, 100% cotton 83 N.A. N.A. 200-250 500 20 1 300 N0. 2 Filrnat, 100% cotton 83 N.A. N.A. 250-310 500 18 l 270 Biaxially oriented 1,000 gauge polyester sheet of the Melinex type manufactured by 101 l. 83 2. 53 l 250 500 60 35 1 Burning.

N.A.-not available.

When a filled calender roll is deformed in nips formed with, for example, two steel rolls at diametrically opposed positions on the filled roll, some of the deformation work which is done on the soft roll on the ingoing side of each nip is converted into energy, in this case mainly heat. The rest of the energy is recaptured as driving power on the outgoing side of the nip as the soft roll resiles to its original form. The part of the deformation energy which is lost per cycle is termed deformation work hysteresis loss, H. The hysteresis loss for normal paper filled rolls has been determined to be between 20% and 50%, and for rolls filled with biaxially oriented plastic sheets to be about 5%. In rolls made of some biaxially oriented polymer sheets, even lower hysteresis losses are encountered. Table I shows the superior physical properties obtained with a calender roll of the invention by comparing them with different kinds of conventional paper-filled rolls.

Neither common commercially available untreated nonoriented plastic sheets nor normal plastic materials in solid form show the low hysteresis losses which characterize biaxially oriented crystalline sheets. The explanation for this is that when a plastic sheet material is stretched in two directions at right angles to each other in the plane of the sheet at a high temperature (e.g., 200 C.) at the time of formation and is not allowed to shrink during comparatively rapid cooling, the molecules are oriented in the plane of the film. The ordering of the molecules brought about in this way results in extensive crystallization upon cooling which again gives superior physical properties as compared to non-oriented film. When the orientation is carried out in the way described and, desirably, by stretching equally in both directions, the properties will be equal in all directions in the plane, i.e., balanced biaxially oriented.

It is interesting and revealing to consider, by comparison, the effects of biaxial orientation on the properties of an exemplary polymer sheet material as set forth in Table I immediately below.

TABLE I.COMPARISON OF THE TENSILE STRENGTHS 0F CAST AND ORIENTED POLYPROPYLENE SHEET (PROPAFILM 0 FROM I01) 50 gauge 100 gauge oriented cast Property Units film Density G-ICDJ- 0.91 0.90

Tensile strength Machine direction:

Lb./in. 25,00030,000 6,500 KgJem. 1,750-2, 100 450 Transverse direction:

Lb./in. 25,000-30,000 5,500 Kg./crn. 1,750-2,100 390 Table III shows the elfect of temperature on the nip deformation work hysteresis loss, H, in a calender roll filled with 1000 gauge Melinex S sheet.

1. A calender roll comprising a central core and a roll filling carried by the core and composed of a plurality of discs fitted onto the core and held in compressed relation on the core to form an essentially solid cylindrical body, the discs being made of a polymeric sheet material having a structure in which the molecules are biaxially oriented.

2. A calender roll according to claim 1 wherein the structure of the sheet material is balanced biaxially oriented.

3. A calender roll according to claim 1 wherein the sheet material is a polymer selected from the group consisting of polyesters, polyolefins, polyamides, and polystyrenes.

4. A calender roll according to claim 3 wherein the polymer is a polyester.

5. A calender roll according to claim 4 wherein the, polymer is polyethylene terephthalate.

6. A calender roll according to claim 1 wherein the roll filling has a deformation work hysteresis loss which is less than ten percent.

References Cited Mark, New Polymers, New Problems, Ed. Marburg Lecture, A.S.T.M., 1959.

WALTER A. SCHEEL, Primary Examiner LEON G. MACHLIN, Assistant Examiner 

